EP3526375B1 - Synthesis of core-shell nanoparticles and uses of said nanoparticles for surface-enhanced raman spectroscopy - Google Patents

Description

Technical area

The invention relates to the synthesis of metallic nanoparticles of the heart-shell type, using a bottom-up approach, from a colloidal solution.

The invention finds advantageous applications for the production of substrates for surface enhanced Raman spectroscopy ( Surface Enhanced Raman Scattering SERS).

Definitions

By “substrate for surface-enhanced Raman spectroscopy”, here is meant a support, for example made of silicon, on which is placed a solution containing metallic nanoparticles.

By “nanoparticle” here is meant a set of polyatomic buildings of nanometric size, typically varying between 1 and 200 nm, whatever one of the three dimensions.

By “metallic nanoparticles” is meant here in particular the nanoparticles of noble metals, and in particular gold and silver.

By “metallic nanoparticles of the core-shell type”, is meant here nanoparticles formed from a core of a first noble metal, in particular gold, coated with a shell of a second noble metal, in particular silver.

State of the art

The methods of synthesis of metallic nanoparticles can be classified into two groups: "top-down" and "bottom-up".

The “top-down” approaches allow starting from the massive material, then reducing it to increasingly fine compounds, for example by electrochemical means, laser ablation in aqueous phase, grinding of powders, lithography. Gold nanoparticles have been produced by laser ablation in water, or even in acetone, toluene, tetrahydrofuran, acetonitrile. Amendola et al (Synthetic metals, Vol 155, Issue 2, 2005, pp. 283-286 ) describe laser ablation in dimethulsulfoxide (DMSO, CAS 67-68-5), allowing the functionalization of gold nanoparticles by fullerene derivatives.

“Bottom-up” approaches start from the smallest possible chemical entities, such as atoms or molecules that come together, to gradually form a nanoparticle. Bottom-up approaches typically use reduction or decomposition reactions, in the gas, liquid or solid phase, physically (laser pyrolysis, CVD deposition), or chemically (chemical, electrochemical, photochemical reduction, soil processes -gel).

The enhanced surface Raman spectroscopy (SERS) is mainly based on the use of SERS substrates (based on metals in general: Au, Ag, Cu) having unique optical properties, linked to the excitation by light of surface plasmons. Localized Surface Plasmon Resonance LSPR.

Typically, a SERS substrate is in the form of metallic nanoparticles (in solution or deposited on a solid support) or a rough metallic film deposited on a flat surface (glass slide, silicon).

The SERS substrates can be active or passive, the active substrates allowing a selectivity of the substrate, by a functionalization, for example use of an antigen for the detection of an antibody.

The vibrational characteristics of the molecules obtained by Raman scattering are generally very weak, but in the presence of nanoparticles of noble metals, these signals are considerably amplified.

The molecules adsorbed on the surface of metallic nanoparticles undergo a SERS effect, due to the strong electromagnetic field undergone by the molecules near the surface of the metal. Field confinement closest to the metal results from the excitation of localized surface plasmons.

Exalted surface Raman spectroscopy (SERS) has thus become an essential vibrational spectroscopy, opening up to fields as varied as biology, analytical chemistry, identification, conservation and restoration of works of art, or forensics.

SERS applications are typically those in which the concentration of the compound to be detected is low, compared to the sensitivity of Raman (for example identification of contaminants, pollutants, biological fluids during forensic investigations), or when analyzes are difficult without preparation (identification of microorganisms for the food industry or medical hygiene, cell imaging), or even for medical diagnoses.

By way of illustration, the SERS is thus proposed for the detection of drug use (e.g. Nuntawong et al, Detection of methamphetamine / amphetamine in human urine based on surface-enhanced Raman spectroscopy and acidulation treatments, Sensors and Actuators B: Chemical, Volume 239, 2017, Pages 139-146 ), the detection of explosives, pollutants such as pesticides or heavy metals (for example mercury, Sun et al, Recent progress in detection of mercury using surface enhanced Raman spectroscopy- A review, Journal of Environmental Sciences, Volume 39, 2016, Pages 134-143 ).

SERS, long considered a remarkable but purely fundamental physical phenomenon, has gradually established itself as a robust and effective analytical tool to exalt (in the sense of amplifying) several orders of magnitude (10 ³ -10 ⁷ ) Raman signal from molecules.

Technical problems

The production of SERS substrates is complex.

As mentioned above, the Raman exaltation is governed by the excitation of the LSPR plasmons, which supposes a good adequacy between the wavelength of laser excitation (Raman) and the position of the plasmon resonance.

For spherical metal nanoparticles, the position and width of the plasmon band is influenced by the size of the nanoparticles. Furthermore, the mode of oscillation of the surface electrons of a particle is specific to its geometry. An isotropic particle, such as for example a spherical particle, will have a single mode, while a complex or anisotropic particle (for example a rod) will have more than one plasmon resonance mode. A rod-shaped or ellipsoidal particle will have two modes of electron resonance: a transverse component, linked to the diameter, and a longitudinal component, linked to the length of the rod. For metallic nanoparticles in the shape of a rod, the aspect ratio of the nanoparticles (length / diameter) has an influence on the strip plasmon corresponding to the longitudinal component. The dispersion of the nanoparticles also has an influence, by coupling between the resonant modes of neighboring nanoparticles.

The generation of localized surface plasmons, responsible for the increase in the interaction between monochromatic light generated by a laser and a probe molecule, is linked to the presence in the substrate of "hot spots", this expression designating spatial zones of dimensions smaller than the wavelength, a hot spot can be produced near two very close nanoparticles (1 to 4 nm) where the electromagnetic field is localized and intense.

The possibility of producing and controlling hot spots is thus linked to the production of SERS substrates having controlled nanostructures.

The quality of a SERS substrate is measured not only by its amplification performance of the Raman signal (quantified by the exaltation factor EF), but also by its reproducibility of manufacture and response, as well as its stability during time.

The search for optimized SERS substrates is extremely active, because despite great advances in the field, there are still many challenges to be met in terms of (i) intrinsic qualities and (ii) versatility.

The manufacture of SERS substrates by depositing colloids and then drying is an expertise in its own right. In fact, the use of protocols abundantly described in the literature for the preparation of active SERS colloids in no way guarantees that they can reproduce the size and the shapes of the objects. There is an essential need for end users of SERS as an analytical technique, to have ready-to-use substrates, where it will suffice to deposit a drop of the solution to be analyzed.

The development of efficient SERS substrates over a wide range of excitation wavelengths is also part of the challenges of the field.

The methods for synthesizing metallic nanoparticles known in the prior art have many drawbacks, not allowing optimal use in the production of substrates for surface-enhanced Raman spectrometry.

The substrates obtained by lithography and etching on thin layers are reproducible and stable over time, but their manufacture is complex and very expensive.

Colloidal solutions (gold, silver), or sol-gels are less expensive, but can give rise to difficulties, particularly in terms of shelf life before use and reproducibility. Colloids can flocculate and then be resuspended, but also precipitate, and aggregate irreversibly.

The electrochemical reduction involves restrictive conditions, such as the use of constant ultrasound and an electrolysis current, and it is difficult to control the uniformity of the nanoparticles obtained by these methods.

Gold nanoparticles of different shapes have been synthesized electrochemically in the presence of surfactant.

Huang et al (Electrochemical synthesis of gold nanocubes, Materials Letters. July 2006 60 (15): 1896-1900 ) describe the synthesis of cubic gold nanoparticles, by electrolysis (Au anode, Pt cathode) of a solution containing acetone and two surfactants: CTAB (hexadecyltrimethylammonium bromide or cetrimonium bromide): ( CH ₃ ) ₃ N (CH ₂ ) ₁₅ CH ₃ Br, CAS 57-09-0 ) and TTAB (tetradecyltrimethylammonium bromide C ₁₇ H ₃₈ BrN, CAS 1119-97-7 ) .

Ma et al (Synthesis of silver and gold nanoparticles by a novel electrochemical method, Chemphyschem. 2004 Jan 23; 5 (1): 68-75 ) describe the synthesis of gold nanoparticles of spherical shape by electrolysis of an aqueous solution containing PVP (Polyvinylpyrrolidone, CAS 9003-39-8 ) .

Sharma et al (Electrochemical synthesis of gold nanorods in track-etched polycarbonate membrane using removable mercury cathode, Journal of Nanoparticle Research. 2012, Vol. 14 Issue 9, p1-10 ) describe the synthesis of gold nanoparticles in the form of rods, by deposition on a nanoporous polycarbonate membrane.

Yu et al (Gold nanorods: electrochemical synthesis and optical properties, The journal of physical chemistry, 101, 34, pp. 6661-6664 ) describe the synthesis of gold nanoparticles in the form of rods, by electrolysis (Au anode, Pt cathode) of a solution containing acetone, and two surfactants (CTAB and TC8AB tetraoctylamonium bromide CAS 14866-33-2 ).

Syntheses by microwave irradiation, radiolysis, photochemistry and sonochemistry are complex and expensive, and require significant energy input. Radiolysis requires the use of a particle accelerator.

A large number of syntheses by chemical reduction, using a metal salt / solvent / reducing agent / surfactant mixture have been proposed, without it being possible to effectively control the geometry of the nanoparticles obtained. In these mixtures, the salt is the precursor containing the metal, the solvent is polar, aqueous or organic (polyol, toluene), the dipole moment of the solvent being high enough to break the bonds of the salt and dissolve it, the reducing agent reduces the species dissolved metal so that it precipitates, and the surfactant protects the particles by adsorbing on their surface to avoid their agglomeration.

In Turkevich-type syntheses, making it possible to produce quasi-spherical and mono-dispersed gold nanoparticles in suspension in water, tetrachlorauric acid is reduced by citrate, which plays both the role of reducing agent and surfactant.

In Brust-type syntheses, chlorauric acid reacts with tetraoctylammonium bromide (transfer agent) in toluene (solvent) and sodium borohydride (reducing agent).

The invention relates more specifically to the synthesis of metallic nanoparticles, of the heart-shell type, by a bottom-up type approach, by colloidal chemical route, the heart being in the form of rods.

The production of metallic nanoparticles, in particular gold, in the form of rods, is known per se. We can refer for example to the growth mechanism called zipping, described in the document Hebié Electrochemical studies of gold nanoparticles structure / activity correlation, 2013 .

The invention aims in particular to provide metallic nanoparticles of gold / silver, of the heart-shell type, by a bottom-up type approach, by colloidal chemical means, the heart being in the form of sticks, in particular sticks of gold, the silver shell being monocrystalline.

• Bai et al, Acs Applied Materials & Interfaces 2014, 6 (5), 3331-3340 ;
• Cho et al Advanced Materials 2010, 22 (6), 744 ;
• Zhang et al Chemistry of Materials 2016, 28 (8), 2728-2741 ;
• Jing, H et al Nano Letters 2014, 14 (6), 3674-3682 ;
• Pastoriza-Santos, Langmuir 2002, 18 (7), 2888-2894 ;
• Zhuo et al Acs Nano 2015, 9 (7), 7523-7535 ;
• Liu et al Physical Chemistry Chemical Physics 2015, 17 (10), 6819-6826 ; Yang et al Journal of the American Chemical Society 2014, 136 (23), 8153-8156 ;
• Yang et al Chemistry European Journal 2015, 21 (3), 988-992 ;
• Zhang et al Chemistry European Journal 2013, 19 (38), 12732-12738 .

Examples of syntheses of gold-silver core-shell nanoparticles are presented in the following documents:

• Bai et al, Acs Applied Materials & Interfaces 2014, 6 (5), 3331-3340 ;
• Cho et al Advanced Materials 2010, 22 (6), 744 ;
• Zhang et al Chemistry of Materials 2016, 28 (8), 2728-2741 ;
• Jing, H et al Nano Letters 2014, 14 (6), 3674-3682 ;
• Pastoriza-Santos, Langmuir 2002, 18 (7), 2888-2894 ;
• Zhuo et al Acs Nano 2015, 9 (7), 7523-7535 ;
• Liu et al Physical Chemistry Chemical Physics 2015, 17 (10), 6819-6826 ; Yang et al Journal of the American Chemical Society 2014, 136 (23), 8153-8156 ;
• Yang et al Chemistry European Journal 2015, 21 (3), 988-992 ;
• Zhang et al Chemistry European Journal 2013, 19 (38), 12732-12738 .

Khlebtsov et al (Au @ Ag core / shell cuboids and dumbbells: Optical properties and SERS response, Journal of Quantitative Spectroscopy and Radiative Transfer, Volume 167, 2015, Pages 64-75 ) describe the synthesis of gold / silver core-shell nanoparticles, for the production of SERS substrates, gold sticks placed in a CTAC solution (hexadecyltrimethylammonium chloride CH ₃ (CH ₂ ) ₁₅ N (Cl) (CH ₃ ) ₃ , CAS 112-02-7), forming the heart, the deposit of the silver shell being obtained by addition of silver nitrate, ascorbic acid and sodium hydroxide.

The document WO 2016/046645 describes the synthesis of asymmetric gold nanoparticles, by reaction of gold rods in the presence of a solvent such as water, a precursor (HAuCl ₄ ), a ligand such as a thiol (4 -mercaptophenol) and a reducing agent (ascorbic acid), a high concentration of ligand inhibiting the growth of gold by epitaxy on rods, and promoting asymmetric growth. The applications mentioned are the capture of solar energy and the production of substrates for SERS.

Objects of the invention

A first object of the invention is to solve the problems presented above, by offering an original synthesis pathway of metallic core-shell nanoparticles, in particular gold / silver, this synthesis being economical, and allowing the production of a large variety of nanoparticle shapes, in a controlled manner.

Another object of the invention is to provide a colloidal solution of metallic nanoparticles, suitable for the production of substrates for enhanced SERS surface Raman spectroscopy.

Another object of the invention is to provide a SERS sensor, in particular for a pollutant such as atrazine.

General presentation of the invention

For these purposes, there is proposed, according to a first aspect, a process for the synthesis of heart-shell gold-silver nanoparticles, from a colloidal aqueous solution of gold seeds with surfactant, the heart-shell gold-silver nanoparticles. being produced from anisotropic gold seeds, this process comprising the addition in the colloidal aqueous solution of a silver precursor and a reducing agent, to deposit silver on the gold seeds in a so-called step main, the method having an incubation step of the colloidal aqueous solution containing the gold seeds with surfactant in a water + DMSO mixture, before the main step carried out with the resulting colloidal aqueous solution of gold (gold seeds + water + DMSO + surfactant).

There is thus proposed a process for the synthesis of heart-shell gold-silver nanoparticles, from a colloidal aqueous solution of gold seeds with surfactant, according to claim 1.

In some implementations, the third duration ranges from a few minutes to around twenty hours.

DMSO (dimethyl sulfoxide, CAS 67-68-5) is a dipolar aprotic solvent. This type of solvent has charges by mesomeric effect and coordinates on a CFC (111) mesh of gold. As a result, this layer can poison the surface of gold, thereby inhibiting the growth of silver. The nano tips rods, essentially constituted by facets (111) can be coordinated by the co-solvent, thus significantly disturbing the diffusion towards the surface and the deposit of silver on these facets

An aprotic solvent does not have an acidic hydrogen atom. A dipolar aprotic solvent cannot form a hydrogen bond, and their molecules act like dipoles.

The dipolar aprotic solvents are for example DMSO (dimethylsulfoxide, CAS 67-68-5), DMF (Dimethylformamide CAS 68-12-2), acetonitrile (CH ₃ CN, CAS 75-05-8), NMP (N-methyl-2-pyrrolidone, CAS 872-50-4).

• le ratio entre le volume de DMSO et le volume total d'eau est inférieur à 2 ; on entend par volume total de l'eau le volume apporté par la solution aqueuse colloïdale contenant les germes d'or avec tensioactif et par l'eau présente dans le mélange eau+DMSO ;
• le ratio entre le volume de DMSO et le volume total d'eau est inférieur ou égal à 0.33 ; avantageusement le ratio est autour de ou égal à 0,33, ce qui permet d'avoir essentiellement des germes d'or bien dispersés et d'incorporer un seul germe d'or dans une coquille d'argent ;
• le ratio entre le volume de DMSO et le volume total d'eau est compris entre 0.33 et 1 ; avantageusement le ratio est autour de ou égal à 1, ce qui permet d'avoir essentiellement des assemblages des germes d'or pointe contre pointe (tête contre tête) enveloppés ensuite par une coquille d'argent ;
• le ratio entre le volume de DMSO et le volume total d'eau est compris entre 1 et 1.5 ; avantageusement le ratio est autour de ou égal à 1,5, ce qui permet d'incorporer des germes d'or face contre face FF dans une coquille d'argent ;
• le ratio entre le volume de DMSO et le volume total d'eau est supérieur à 1.5 ; ce qui permet à nouveau d'avoir essentiellement des germes d'or bien dispersés et d'incorporer un seul germe d'or par coquille d'argent, comme représenté sur la figure 9 ;
• le procédé comprend successivement
  • ∘ l'étape d'incubation de la solution aqueuse colloïdale contenant les germes d'or avec tensioactif dans le DMSO, pendant une première durée donnée ;
  • ∘ une étape d'ajout d'un autre tensioactif ;
  • ∘ une étape de chauffage, pendant une deuxième durée donnée ;
  • ∘ l'étape principale de réaction des germes d'or avec le précurseur d'argent et le réducteur dans la solution aqueuse colloïdale de germes d'or ;
  • ∘ après une troisième durée donnée, une étape de centrifugation du mélange ;
• l'étape de réaction comprend l'ajout de tensioactif avec le réducteur ;
• le tensioactif peut être du chlorure de cetyltrimethylammonium (CTAC) et/ou de bromure de cetyltriméthylammonium (CTAB) ;
• le réducteur est l'acide ascorbique (AA) ;
• le précurseur d'argent est du nitrate d'argent.

According to various implementations, the method has the following characteristics, if necessary combined:

• the ratio between the volume of DMSO and the total volume of water is less than 2; the total volume of water is understood to mean the volume provided by the colloidal aqueous solution containing the golden germs with surfactant and by the water present in the water + DMSO mixture;
• the ratio between the volume of DMSO and the total volume of water is less than or equal to 0.33; advantageously the ratio is around or equal to 0.33, which makes it possible to have essentially well dispersed gold seeds and to incorporate a single gold seed in a silver shell;
• the ratio between the volume of DMSO and the total volume of water is between 0.33 and 1; advantageously the ratio is around or equal to 1, which makes it possible to have essentially assemblies of gold seeds point against point (head against head) then wrapped by a silver shell;
• the ratio between the volume of DMSO and the total volume of water is between 1 and 1.5; advantageously the ratio is around or equal to 1.5, which makes it possible to incorporate gold seeds face to face FF in a silver shell;
• the ratio between the volume of DMSO and the total volume of water is greater than 1.5; which again makes it possible to have essentially well dispersed golden germs and to incorporate a single golden germ per silver shell, as shown in the figure 9 ;
• the process successively comprises
  • ∘ the stage of incubation of the colloidal aqueous solution containing the golden germs with surfactant in DMSO, for a first given duration;
  • ∘ a step of adding another surfactant;
  • ∘ a heating stage, for a second given duration;
  • ∘ the main stage of reaction of the gold seeds with the silver precursor and the reducing agent in the colloidal aqueous solution of gold seeds;
  • ∘ after a third given duration, a step of centrifuging the mixture;
• the reaction step comprises adding surfactant with the reducing agent;
• the surfactant can be cetyltrimethylammonium chloride (CTAC) and / or cetyltrimethylammonium bromide (CTAB);
• the reducing agent is ascorbic acid (AA);
• the silver precursor is silver nitrate.

A second aspect is proposed, gold-silver core-shell nanoparticles produced from elongated gold seeds, obtained by the process as presented above, the gold-silver core-shell nanoparticles having at minus two germs of gold per heart for a silver shell enveloping the heart.

In one implementation, the germs of gold are placed head against head (TT) in the silver shell.

In another implementation, the seeds of gold are arranged face to face (FF) in the silver shell.

Advantageously, the germs of gold are nano sticks.

According to another aspect, a colloidal solution is proposed comprising gold-silver core-shell nanoparticles as presented above.

According to another aspect, a substrate for surface-enhanced Raman spectroscopy (SERS) is proposed comprising gold-silver core-shell nanoparticles as presented above.

Advantageously, such a substrate is applied to the detection of organic pollutants.

In one implementation, the organic pollutant is atrazine (CAS 217-617-8).

List of Figures

• la figure 1 est une vue en microscopie électronique à balayage de nanoparticules d'or, en forme de nano bâtonnets de différents rapports d'aspect (rapport de la plus grande dimension sur la plus petite dimension);
• les figure 2-A et 2-B présentent une vue en microscopie électronique à balayage de nanoparticules cœur-coquille or/argent, selon une mise en oeuvre du procédé selon l'invention ;
• la figure 3 est une vue en microscopie électronique à balayage de nanoparticules cœur-coquille or/argent résultant de la surcroissance d'argent sur des nanoparticules cœur-coquille or/argent, selon une autre mise en œuvre du procédé selon l'invention ;
• la figure 4 est un spectre d'absorption des solutions de nanoparticules d'or pour différents ajouts de DMSO ;
• la figure 5-A est un graphe représentant l'évolution du spectre d'absorption des solutions colloïdales au cours du temps après addition de nitrate d'argent et d'acide ascorbique ; la figure 5-B est un graphe représentant l'évolution de la longueur d'onde du maximum de la résonance longitudinale en fonction du temps, suite à l'ajout de nitrate d'argent et d'acide ascorbique dans des solutions contenant des nanoparticules d'or, avec emploi de différents co-solvants (MeCN, DMSO, EtOH), ou sans l'emploi de co-solvant ;
• les figures 6 et 7 sont des vues en microscopie électronique à balayage d'arrangements de nanoparticules obtenues selon un procédé de synthèse selon l'invention ;
• la figure 8 est une représentation schématique du procédé de synthèse selon les proportions de DMSO dans le milieu réactionnel et les particules cœur-coquille résultantes ;
• la figure 9 est une représentation schématique de l'effet du DMSO sur les propriétés des nano particules d'or, avec ici DMSO = (volume de DMSO)/(Volume total) avec Volume total=Volume en eau + Volume en DMSO.

Other objects and advantages of the invention will appear in the light of the description of an embodiment, given below with reference to the appended drawings in which:

• the figure 1 is a view in scanning electron microscopy of gold nanoparticles, in the form of nano sticks with different aspect ratios (ratio of the largest dimension to the smallest dimension);
• the figure 2-A and 2-B present a view in scanning electron microscopy of gold / silver core-shell nanoparticles, according to an implementation of the method according to the invention;
• the figure 3 is a scanning electron microscopy view of gold / silver core-shell nanoparticles resulting from the overgrowth of silver on gold / silver core-shell nanoparticles, according to another implementation of the method according to the invention;
• the figure 4 is an absorption spectrum of solutions of gold nanoparticles for different additions of DMSO;
• the figure 5-A is a graph representing the evolution of the absorption spectrum of colloidal solutions over time after the addition of silver nitrate and ascorbic acid; the figure 5-B is a graph representing the evolution of the wavelength of the maximum of the longitudinal resonance as a function of time, following the addition of silver nitrate and ascorbic acid in solutions containing gold nanoparticles, with use of different co-solvents (MeCN, DMSO, EtOH), or without the use of co-solvent;
• the figures 6 and 7 are views by scanning electron microscopy of nanoparticle arrangements obtained according to a synthesis method according to the invention;
• the figure 8 is a schematic representation of the synthesis process according to the proportions of DMSO in the reaction medium and the resulting core-shell particles;
• the figure 9 is a schematic representation of the effect of DMSO on the properties of nanoparticles of gold, with here DMSO = (volume of DMSO) / (Total volume) with Total volume = Volume in water + Volume in DMSO.

Detailed description of embodiments

In a first step, a synthesis of nano gold sticks is carried out, according to a conventional method.

As indicated, the synthesis of nano gold rods, in colloidal solution, is known per se, and is not described again here.

The nano gold sticks are of general shape represented in figure 1 . As an indication, three families of nano-rods used in the syntheses are presented from left to right on the Figure 1 for aspect ratios of 4.0 (55x14 nm), 2.0 (76x37 nm) and 2.4 (96x40 nm). The quantities in parentheses expressed in nanometers represent, in order, the average length along the major axis of the stick and along the minor axis.

The nanorods of gold are incubated in a dipolar aprotic solvent.

In one implementation, this solvent is dimethyl sulfoxide DMSO.

By way of example, the nano-gold bars (120 μl at 1.82nM) are incubated in 100 μl of dimethyl sulfoxide DMSO for 30 min.

Then, the nanorods of gold are transferred into a solution of CTAC (cetyltrimethylammonium chloride) and heated.

For example, the nano sticks are transferred to a CTAC solution (26mg in 3.8mL of water) and heated at 60 ° C for 20min.

The silver deposition is initiated, by adding to the previous preparation, a solution of silver nitrate AgNO ₃ , and a solution containing ascorbic acid AA and CTAC.

As an example, the deposition of silver is initiated by adding a solution of silver nitrate AgNO ₃ (2mM, 1.2mL), and a solution containing ascorbic acid (AA, 45mM, 1.2 mL) and CTAC (40mM).

A complete preparation protocol P is as follows, in one embodiment.

In a tube containing 300 μl of nano gold rods (C = 0.15nM), 100 μL of DMSO are introduced, and the solution is left in the rest for 5 minutes, then added to an aqueous solution of CTAC (26 mg of CTAC in 4.6 ml of Milli-Q water) previously heated to 60 ° C. The resulting solution is stirred in a water bath at 60 ° C for 20 min. The deposition step is started by adding 1.2mL of a 2mM AgNO ₃ solution and 1.2mL of a AA (44mM) / CTAC (40mM) solution protected from light. The addition is done dropwise with moderate stirring. The silver deposit is stopped after 100 min, by centrifuging at 6000 rpm for 8 min.

Another preparation protocol makes it possible to work on a larger scale, as a first step towards an industrialization of the synthesis process. This process, allowing one implementation to produce up to 1000 times more core-shell gold-silver particles, is as follows:
1.15mL of DMSO are introduced into 16 tubes, each containing 150 μL of nano gold sticks (C = 17nM), and the solution is left to stand for 10 minutes. The container of the 16 tubes is added to an aqueous solution of CTAC (320 mg of CATC in 22 ml of water) previously heated to 60 ° C. The resulting solution is maintained at 60 ° C and with stirring for 20 min. The deposition step is started by adding 10mL of a 4mM AgNO ₃ solution and 10mL of a AA (100mM) / CTAC (80mM) solution protected from light. The addition is done dropwise with moderate stirring. The silver deposit is stopped after a period of between 90 min and 20 hours (depending on the desired particle form), by centrifuging at 6000 rpm for 5 min.

The shape of the final object and the thickness of silver deposited are modulated by the proportion of DMSO in the mixture, the quantity of silver precursor and the reaction time before centrifugation of the mixture.

The shape of the gold / silver heart-shell nanoparticles is that of a warhead, as it appears in figure 2 .

The deposit of silver is done essentially along the minor axis and the tips of the gold rods are flush with the surface of the heart-shells. These anisotropic deposits in the two directions generate a reduction in the aspect ratio, which is 1.4 for the core-shell nanoparticles, compared to the ratio of 3.9 for the starting nano-gold sticks.

The deposit of silver on gold is monitored by UV-visible extinction spectroscopy, with a gradual shift of the longitudinal plasmon resonance towards blue.

The optical properties of the core-shell nanoparticles obtained, the position of the extinction band corresponding to the longitudinal plasmon resonance of the nanoparticles is adjustable between 510 and 800 nm, depending on the thickness of the silver deposit and the aspect ratio starting gold germ.

Taking advantage of the localized surface plasmon resonance of the synthesized gold / silver nanoparticles and the best expected enhancement for silver (compared to gold alone), characterizations in surface enhanced Raman spectroscopy (SERS) were carried out in solution in the presence of molecular probes.

The spectra show an enhancement of the Raman signal between 4 and 100 times, compared to that recorded for gold / silver core-shell nanoparticles obtained in water without co-solvent.

By this synthesis, the gold / silver core-shell nanoparticles obtained by the process according to the invention contain traces of DMSO visible by spectroscopy.

Once isolated and deposited on a solid support (silicon wafer for example), the heart-shells obtained spontaneously self-organize in a 3D network, 2D structure or in a 1D chain, with lengths (or surfaces) greater than 1 micrometer, depending the concentration of the solution deposited and the shape of the core-shell nanoparticles. The figures 6 and 7 show some examples of arrangements obtained.

As illustrated on figures 6 and 7 , the gold / silver core-shell nanoparticle (NP) networks organized in 1D, 2D or 3D have a long-distance order ranging from a few microns to a few tens of microns. For 2D and 3D arrangements, they can have surfaces of at least 10 square micrometers, advantageously 40 square micrometers, or even larger, and which are repeated on the substrate. These large surfaces obtained can be explained in particular by the fact of the great homogeneity of form of the NPs obtained by the process.

As illustrated in the 3D structure of the figure 6 , NP show white colors, a little gray or more gray, depending on their distance under the microscope.

These networks are obtained in particular with the gold / silver core-shell nanoparticles having a single gold germ surrounded by a silver shell.

Depending in particular on the concentrations of surfactants used, of gold-silver core-shell nanoparticles, of thickness of the silver layer, of the size of the gold germs, certain arrangements are obtained rather. For example, gold seeds coated with a weak deposit of silver (less than 5 nm) give rather 1D chains, while gold-silver core-shell particles with a thicker layer of silver (10 nm and more) rather organize themselves in 2D or 3D networks. Furthermore, these ordered domains will be distant on the silicon substrate if the deposit is made from dilute solutions of core-shell gold-silver nanoparticles in water.

Influence of the amount of co-solvent

• maintenant le type du germe d'or de départ et sa quantité, la quantité du réducteur et du précurseur d'argent, le temps de la réaction avant centrifugation ;
• variant le rapport DMSO/H₂O

Tests have been conducted, in

• maintaining the type of starting gold germ and its quantity, the quantity of the reducing agent and the silver precursor, the reaction time before centrifugation;
• varying the DMSO / H ₂ O ratio

According to the proportion of DMSO added to the medium, three limiting behaviors were identified in solution, by absorption spectroscopy, as illustrated in figure 4 .

For DMSO / H ₂ O ratios less than 0.33 (here volume ratio = DMSO volume / total volume of water), the stability of the gold nano-sticks is maintained in solution and the latter remain at a distance from each other. others.

For a DMSO / H ₂ O ratio of 1 in the starting solution, a point-to-point solution assembly of the gold nano-sticks is highlighted ( figure 9 ). This assembly is detected by the appearance in short time of a band, shifted towards the red with respect to the longitudinal plasmon resonance band of the starting nano-rods. By assembly point against point (or head against head TT), one indicates here the observation of the bringing together of nano-sticks by their end parts, the contiguous nano-sticks being aligned with each other or forming an angle between them.

Finally, for a higher ratio of DMSO / H ₂ O (1.5) in the starting solution, the kinetic monitoring by absorption spectroscopy over time, shows an increasing shift towards the blue of the longitudinal resonance and an increasing shift towards the red of the transverse resonance. These spectral modifications are characteristic of a face-to-face (FF) assembly of the nanoparticles ( figure 9 ). By face-to-face assembly, we here designate the observation of the bringing together of the nano-sticks by their lateral faces, the directions of pitch of the contiguous nano-sticks being substantially parallel.

The figure 9 illustrates the different states of the nano gold rods, according to the percentage of DMSO (proportion of DMSO in the water + DMSO mixture).

At a value of 50% of DMSO, the nano gold rods are assembled in point to point solution.

At a value of 60% of DMSO, the nano gold rods appear assembled in solution in pair face to face.

At a value of 25% (or less than 25%) of DMSO, the nanorods of gold appear dispersed in solution. For proportions of DMSO between these characteristic values, the nano sticks of gold are in the form of populations of two types, for example assembled face against face and point against point, for proportions of DMSO between 50 and 60% .

For percentages of DMSO in the mixture of between 80 and 95%, the gold nanoparticles again appear dispersed in solution and the nanoparticles produced thereafter have a single gold germ per silver shell.

• d'orienter sélectivement la croissance de l'argent sur la surface d'or, et/ou
• d'assembler les germes d'or en solution face-contre-face (FF) ou tête-contre-tête (TT), ou de les laisser isolés les uns des autres, avant de faire croitre la couche d'argent tout autour.

Thus, the use of a water-DMSO mixture as described in the invention allows:

• to selectively direct the growth of silver on the surface of gold, and / or
• assemble the golden sprouts in face-to-face (FF) or head-against-head (TT) solution, or leave them isolated from each other, before growing the layer of silver all around.

The particles after incubation in the water-DMSO mixture retain their layer of surfactants without which the particles would irreversibly precipitate, making the mixture unusable. Adding DMSO plays two roles. DMSO allows on the one hand to modify the organization of the surfactant (by playing on the zeta potential of the colloidal suspension) on the surface of the gold germs, and on the other hand, is adsorbed on certain facets of the germs. however as revealed by studies by SERS. Surfactants (CTAB, CTAC) are always present on the surface of germs, but adopt a different organization in the presence of DMSO.

The origin of the observed phenomena is not completely understood, the following hypotheses being put forward.

DMSO is a dipolar aprotic solvent, highly dissociative in nature and capable of solubilizing polarizable, polar and ionic compounds.

This type of solvent has charges by mesomeric effect and coordinates on a particular face (111) of the gold single crystal (CFC) of gold, by ionic interaction between its negatively charged oxygen and one or two ad-atoms. gold positively charged. As a result, this layer can poison the surface of gold, thereby inhibiting the growth of silver. The tips of the nanorods, essentially constituted by facets (111) can be coordinated by the DMSO, thus significantly disturbing the diffusion towards the surface and the deposit of silver on these facets. The coordination of DMSO on gold would take place via the two possible coordination modes for this ligand (S-donor or O-donor).

Comparison of the effects of different co-solvents

• acétonitrile (MeCN, CAS 75-05-8), solvant polaire aprotique ;
• éthanol (EtOH, CAS 64-17-5), solvant polaire protique ;
• tétrahydrofurane (THF, CAS 109-99-9), solvant polaire aprotique.

Tests have been carried out to compare the effects of DMSO with that of other co-solvents:

• acetonitrile (MeCN, CAS 75-05-8), polar aprotic solvent;
• ethanol (EtOH, CAS 64-17-5), polar protic solvent;
• tetrahydrofuran (THF, CAS 109-99-9), polar aprotic solvent.

The previously presented protocol P was applied, by replacing DMSO with ethanol, or by replacing the volume of DMSO with 75 μl of MeCN and 25 μl of Milli-Q water, or by replacing the volume of DMSO with l Milli-Q water.

Different silver deposits were made on germs of gold pre-incubated in a water / co-solvent mixture (subsequently identified as NRsEtOH, NRsMeCN and NRsDMSO respectively for a mixture with EtOH, MeCN and DMSO).

These deposits were compared to the same deposit of silver on gold nanowires incubated in water only.

The kinetic monitoring of the reaction media was carried out by UV-Visible absorption spectroscopy, as presented in Figure 5-A .

The variations in the wavelength of the maximum of the longitudinal resonance band (ΔLLSP) as a function of time show a shift towards the blue of this band, correlated to a deposit of silver along the short axis of the rod.

After 45 min, the largest LLSP shift is recorded for nano-gold bars incubated in DMSO ((ΔLLSP) DMSO = 227nm, (ΔLLSP) H2O, MeCN = 197nm, (ΔLLSP) EtOH = 183nm); as shown in figure 5-B .

The speed of depositing money on these NRsDMSO is faster than that of depositing money on NRsMeCN, NRsEtOH and NRsH ₂ O (used as a reference).

The disruption of the surfactant layer, by adding organic solvent such as ethanol or acetonitrile, does not directly affect the shape of the final core-shells. The modifications are limited to slight differences in the kinetics of the deposition of silver on gold and in the wavelength of the longitudinal resonance of objects, attributed to the difference in solubility of the surfactant present on the surface of gold in water, ethanol and acetonitrile.

On the other hand, for DMSO, kinetic monitoring shows a shift towards the blue of the longitudinal and transverse resonances, and of the narrower and more intense resonances. These spectral modifications may be due to a larger deposit of silver or to morphological differences in the objects obtained.

Silver overgrowth tests on gold / silver core-shells to give particular shapes to nanoparticles

Silver overgrowth tests on heart-shells incubated in DMSO were carried out. A blue shift of the wavelength of the longitudinal resonance of the starting objects is found. SEM images show that the deposition is essentially along the minor axis and in one direction only, and the final objects look like pyramids, the base of which is a nano-stick of gold, as illustrated in figure 3 .

Deposit of money on dimers

We have highlighted the role of DMSO in the assembly of nano gold rods in dimers face to face FF for a very specific water / DMSO ratio, as well as its effect on the final shape of the core-shell particles. Thus, the addition of DMSO in the colloidal solution so that the DMSO / H ₂ O ratio is equal to 1.5, gives rise to this FF assembly of the starting nanorods of gold.

Note that the addition of surfactant during the incubation stage makes it possible to stop the assembly of the golden germs either head to head (TT) or face to face (FF) (depending on the ratio DMSO volume to volume water). Thus, depending on the time chosen for this addition, it can be made assemblies with two, three, four etc ... seeds of gold for the heart then wrapped by the silver shell.

This assembly is highlighted by an increasing shift towards the blue of the longitudinal resonance and an increasing shift towards the red of the transverse resonance.

The assembly of Au NRs in FF configuration is probably due to the (attractive) depletion forces, which destabilize the colloidal solution. It is the solubilizing power of DMSO which is brought into play in this phenomenon of partial desorption of the surfactant from the surfactant bilayer around the particles.

The reduction in the electrostatic repulsion between the rods, in addition to the attractive Van der Waals interactions between the hydrophobic chains thus exposed will be the driving force of the assembly in dimer at first, then in oligomers at long times.

To freeze the system in the dimer state and avoid the formation of oligomers, the concentration of CTAC surfactant in the medium is suddenly increased beyond the CMC _CTAC .

Indeed, the addition of excess CTAC reform the CTAC bilayer with the hydrophobic chains exposed around the dimers and avoid any aggregation by electrostatic repulsion.

The encapsulation of the FF dimers in a silver layer was carried out by reduction of the AgNO ₃ precursor with ascorbic acid AA in the solution of dimers at 60 ° C.

A protocol for manufacturing core-shell dimer nanoparticles is as follows, in an implementation: at 100 μL of a solution of nano sticks of NRsAu gold (4nM), a volume of 150 μL of DMSO is added and the assembly facing against the face of the nano gold sticks is stopped by adding 800µL of CTAC 8mM. The solution is centrifuged and redispersed in 5mL of 16mM CTAC. To trigger the deposition, 1.2mL of the 2mM AgNO ₃ aqueous solution and 1.2mL of the AA (44mM) / CTAC (40mM) aqueous solution are added, protected from light. The addition is done dropwise with moderate stirring. The deposition is stopped after 60 min by centrifuging at 6000 rpm for 8 min.

The silver deposit is revealed during the follow-up by absorption spectroscopy, by an offset of at least 100nm towards the blue of the LLSP resonance after only 3 minutes of reaction.

The core-shell particles obtained are in the form of octahedra, with more pronounced tips, which give them more interesting properties of exaltation, as shown in figure 3 .

Applications

The studies compared by SERS of particles obtained by this new synthetic route, show an increase in the SERS gain, compared to the cuboid particles conventionally obtained in water. The SERS substrates of the invention exhibit greater homogeneity in terms of response compared to those of the state of the art, due to the greater homogeneity of arrangement of the core-shell-gold-silver nanoparticles deposited.

The gold-silver core-shell nanoparticles obtained (NRsAu @ Ag) with more pointed ends will induce stronger confinement from the field to the tips.

Application to the detection of atrazine (ATR)

At 183 μl of basic Milli-Q water (pH equal to 6 adjusted by the addition of 0.1M NaOH), a volume x of ATR (1, 0.1, 0.01, 0.001mM in acetonitrile) and y of β- CD or β-cyclodextrin (6, 0.66, 0.066, 0.0066 mM in water) are added. The solutions prepared beforehand are added to 100 μl of NRsAu @ Ag. The values of x and y (between 4 and 16µl) as well as the concentrations of β-CD and ATR are chosen such that the final concentrations of ATR and β-CD are equal to 0.05, 1, 3, 6, 12.5, 25, 100, 200µM. The solutions are incubated for 1 hour before characterizing them by SERS. Alpha-cyclodextrin can also be used to detect atrazine.

Advantages of the invention

The invention has many advantages.

The invention allows the modulation of the shapes and of the self-assembly of gold-silver core-shell nanoparticles, by an original synthesis route in water / organic co-solvent medium.

This synthesis process makes it possible to include one or more seeds of gold of elongated shape (nano-stick of dimension ranging from 15 to 150 nm), in a matrix of monocrystalline silver.

It is possible to incorporate more than one gold nano-stick germ. In fact, the relative water / DMSO proportions induce, for certain compositions, an assembly of the gold nanorods face against face, during the incubation step.

This process is remarkably simpler than those commonly used in the literature, to obtain more exotic forms than conventional cuboids.

During the overgrowth of silver on gold, the shape of the core-shell particles is controlled, by the kinetics of deposition of silver atoms on the gold surface.

The invention has several advantages for the manufacture of SERS substrates.

The bottom-up preparation approach makes it possible to modulate the size, shape and therefore the position of the plasmon resonance of the substrate, making it possible to envisage several wavelengths of laser excitation (532, 632.8 and 785 nm).

In addition, the shell of silver nanoparticles is an advantage in terms of enhancement factor, since silver is known to be more effective than gold (inter-band responsible for losses by damping of the plasmon located in the visible for the gold and in UV for silver).

In addition, the self-assembly on solid substrate of the nanoparticles obtained by the synthesis process, opens up to SERS sensor applications.

A bottom-up approach to manufacturing these SERS substrates from colloids constitutes an alternative to top-down approaches by physical lithography, which although expensive are currently preferred for commercial substrates (better reproducibility). One of the disadvantages of colloids, compared to lithographic substrates, is the more random and non-reproducible nature of the location of hot spots. The self-assembly mechanisms of the nanoparticles allow greater consistency of the properties of the substrates.

• agroalimentaire: par exemple pour la détection de mélamine dans le lait ;
• police scientifique/ sécurité: drogue, poison, explosifs ;
• analyse environnementale des contaminants dans l'eau: PCB, atrazine, pesticide, hormones ;
• biocapteurs (diagnostic précoce de marqueurs).

The spontaneous arrangements on the surface of the core-shell nanoparticles make it possible to develop new SERS substrates, for sensor applications, for example for the following applications:

• food industry: for example for the detection of melamine in milk;
• forensics / security: drugs, poison, explosives;
• environmental analysis of contaminants in water: PCB, atrazine, pesticide, hormones;
• biosensors (early marker diagnosis).

The applications envisaged are in the field of chemical sensors using surface-enhanced Raman spectroscopy, as a tool for quantifying and identifying the analyte of interest.

Claims (23)

1. A method for synthesising gold-silver core-shell nanoparticles, from a colloidal aqueous solution of gold seeds with a surfactant, the gold-silver core-shell nanoparticles being made from anisotropic gold seeds, characterised in that the method successively comprises:

   - a step of incubating the colloidal aqueous solution containing gold seeds with a surfactant, into a water and DMSO solvent mixture, for a first given duration; to modify organisation of the surfactant and assembling of gold seeds;

   - a step of adding a surfactant into the previous resulting mixture;

   - a step of heating the resulting mixture for a second given duration;

   - a step of adding a silver precursor and a reducing agent into the resulting mixture, to perform silver deposition onto the gold seeds, in a so-called main step, for a third given duration;

   - a step of extracting nanoparticles.
2. The method according to claim 1, characterised in that the ratio of the DMSO volume to the total water volume is lower than 2 and greater than 0.1, the total water volume being the volume brought by the colloidal aqueous solution of gold seeds with a surfactant and by water present in the incubating water + DMSO solvent mixture.
3. The method according to one of claims 1 to 2, characterised in that the ratio of the DMSO volume to the total water volume is lower than or equal to 0.33, the total water volume being the volume brought by the colloidal aqueous solution of gold seeds with a surfactant and by water present in the incubating water + DMSO solvent mixture.
4. The method according to one of claims 1 to 2, characterised in that the ratio of the DMSO volume to the total water volume is 1, the total water volume being the volume brought by the colloidal aqueous solution of gold seeds with a surfactant and by water present in the incubating water + DMSO solvent mixture.
5. The method according to one of claims 1 to 2, characterised in that the ratio of the DMSO volume to the total water volume is 1.5, the total water volume being the volume brought by the colloidal aqueous solution of gold seeds with a surfactant and by water present in the incubating water + DMSO solvent mixture.
6. The method according to claim 1, characterised in that the ratio of the DMSO volume to the total water volume is lower than 10 and greater than 2, the total water volume being the volume brought by the colloidal aqueous solution of gold seeds with a surfactant and by water present in the incubating water + DMSO solvent mixture.
7. The method according to claim 6, characterised in that the ratio of the DMSO volume to the total water volume is greater than or equal to 4, the total water volume being the volume brought by the colloidal aqueous solution of gold seeds with a surfactant and by water present in the incubating water + DMSO solvent mixture.
8. The method for synthesising core-shell nanoparticles according to one of the previous claims 1 to 7, characterised in that the step of adding a surfactant takes place after a first optimum incubating duration ranging from a few minutes to one hour and determined by spectroscopy, for controlling the assembling of gold seeds and for obtaining either one gold seed per silver shell, or two gold seeds per silver shell.
9. The method according to one of claims 1 to 8, characterised in that the extracting step is performed by centrifugation.
10. The method according to any of the previous claims, characterised in that the surfactant is selected from the following list: cetyltrimethylammonium chloride (CTAC), cetyltrimethylammonium bromide (CTAB) or benzyldimethylhexadecylammonium chloride (BDAC).
11. The method according to any of the previous claims, characterised in that, after the extracting step, the method has a new step of adding a silver precursor and a reducing agent, on the nanoparticles, to perform silver overgrowth.
12. The method according to any of the previous claims, characterised in that the reducing agent is an ascorbic acid (AA) and surfactant solution, and in that the silver precursor is silver nitrate.
13. Gold-silver core-shell nanoparticles made from elongate-shaped gold seeds, obtained by the method defined according to one of the previous claims 1 to 12, and having DMSO traces visible by spectroscopy.
14. The gold-silver core-shell nanoparticles according to claim 13, characterised in that the gold-silver core-shell nanoparticles each have at least two gold seeds per core, encapsulated into the same silver shell wrapping the core.
15. The gold-silver core-shell nanoparticles according to claim 14, characterised in that, for each nanoparticle, the gold seeds are disposed head to head in the same silver shell.
16. The gold-silver core-shell nanoparticles according to claim 14, characterised in that, for each nanoparticle, the gold seeds are disposed face to face in the same silver shell.
17. The gold-silver core-shell nanoparticles according to one of claims 14 to 16, characterised in that the gold seeds are nanorods.
18. The gold-silver core-shell nanoparticles according to claim 17, characterised in that the nanorods have a mean aspect ratio between 2 and 5.
19. A solid substrate for Surface Enhanced Raman Spectroscopy (SERS) comprising gold-silver core-shell nanoparticles from any of claims 13 to 18, the nanoparticles being self-assembled as 2D structure, 3D-array on surface areas greater than 10µm², advantageously greater than 40µm², for each 2D or 3D array.
20. The solid substrate for surface enhanced Raman spectroscopy (SERS) comprising gold-silver core-shell nanoparticles from any of claims 13 to 18, the nanoparticles being self-assembled as a 1D-chain, the 1D-chains having characteristic dimensions ranging from 2 to 3µm.
21. An application of the substrate according to one of claims 19 or 20 for detecting analytes, such as organic pollutants.
22. The application according to claim 21, the analyte being atrazine, the gold-silver core-shell nanoparticles being used with beta-cyclodextrin (CAS 7585-39-9) or alpha-cyclodextrin (CAS 10016-20-3).
23. The application according to claim 21, the analyte being selected from the following list: thiram (CAS 137-26-8), phosmet (CAS 732-11-6), malathion (CAS 121-75-5), (4,4')-BPE (CAS 13362-78-2), 4-mercaptobenzoic acid (CAS 1074-36-8).

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